Megasplay Fault Research Articles

Spatial and temporal evolution of the megasplay fault in the Nankai Trough

The temporal and spatial evolution of a seismogenic megasplay fault in the Kumano area, Nankai Trough (southwest Japan), is revealed by detailed investigation of the three‐dimensional structure of the shallow portions of the fault, combined with the results of drilling and dating of cores from Integrated Ocean Drilling Program (IODP) Expedition 316. The ENE striking eastern portion of the splay fault has remained active since the inception of faulting at ∼1.95 Ma. The recent shortening rate is ∼1 m/kyr, which represents ∼1.5%–2.5% of the total plate convergence rate of ∼40–65 m/kyr. The NE striking western portion of the splay fault exhibits a different mode of activity. Early stage activity (before 1.55 Ma) was similar to the eastern portion, but the fault was inactive between 1.55 and 1.24 Ma. The fault was reactivated for a short time at ∼1.24 Ma but again ceased activity after formation of the secondary branch and has been inactive since 1.24 Ma. Cessation of splay fault activity in the western domain after 1.55 Ma may be due to collision with a seamount and resulting bending of the accretionary prism in the splay fault footwall. Continuous activity of the eastern domain of the splay fault after 1.24 Ma may be related to geometrical favorability due to reorientation of the fault after the seamount passed beneath the imbricate thrust zone, leading to initiation of slightly oblique subduction.

Dynamic changes in fluid redox state associated with episodic fault rupture along a megasplay fault in a subduction zone

Although redox state plays an important role in chemical reactions, it has received little attention in terms of processes of fluid–rock interaction in fault zones. The results of geochemical analyses (major element mapping and trace element analyses) of carbonate veins along the Nobeoka Thrust, a fossilized megasplay fault within the Shimanto Accretionary Complex, Japan, suggest dynamic sudden change in redox state associated with seismic faulting and fluid flow. Fluids migrating along the fault zone were relatively oxidized during the pre-failure fluid pressurization stage of the seismic cycle, but were relatively reduced during the immediate post-failure stage. The generation of a reducing post-failure hydrothermal regime may be associated with seismogenic rupture along the megasplay fault, involving the advection of exotic reducing fluid from deeper levels or in situ generation of hydrogen by co-seismic mechanochemical reactions.

High‐velocity frictional properties of clay‐rich fault gouge in a megasplay fault zone, Nankai subduction zone

We conducted high‐velocity friction experiments on clay‐rich fault gouge taken from the megasplay fault zone in the Nankai subduction zone under dry and wet conditions. In the dry tests, dehydration of clay minerals occurred by frictional heating, and slip weakening is related to thermal pressurization associated with water vaporization, resulting in a random distribution of clay‐clast aggregates in the gouge matrix. In the wet tests, slip weakening is caused by pore‐fluid pressurization via shear‐enhanced compaction and frictional heating, and there is a very weak dependence of the steady‐state shear stress on the normal stress. The resulting microstructure reflects the grain size segregation in a granular‐fluid shear flow at high shear rates. These results suggest that earthquake rupture propagates easily through clay‐rich fault gouge by high‐velocity weakening, potentially leaving the microstructures resulting from the frictional heating or the flow sorting at high slip rates.

Rapid forearc basin uplift and megasplay fault development from 3D seismic images of Nankai Margin off Kii Peninsula, Japan

Offshore Kii Peninsula, Japan, a large thrust within the overriding forearc, the megasplay fault, appears to move coseismically during great earthquakes. 3D seismic images of the Kumano forearc basin that overlies the megasplay, correlated with IODP drilling data, are a potential record of the history of large-scale motion along this structure. In the early Quaternary, uplift occurred in the southwest portion of the basin that may be a preliminary phase of motion along the megasplay. More extensive landward tilting of the outer basin sediments across the seismic volume occurred over ~ 300 kyr in the middle to late Quaternary (1.3–1 Ma); this tilting event may represent the major period of motion along the megasplay that formed the modern fault geometry. Extensive normal faulting that cuts the forearc basin sediments clearly formed subsequent to the late Quaternary tilting and in many cases offset the modern seafloor; these faults may form either due to gravitational response to the uplift or as a by-product of sediment underthusting. These results suggest that the megasplay is a recently formed and transient structure and support the idea that out-of-sequence thrusts serving as the dominant structure for convergence-driven shortening in a subduction zone may be short-lived geologically but dominate a margin during these intervals.

IODP Expedition 319, NanTroSEIZE Stage 2: First IODP Riser Drilling Operations and Observatory Installation Towards Understanding Subduction Zone Seismogenesis

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a major drilling project designed to investigate fault mechanics and the seismogenic behavior of subduction zone plate boundaries. Expedition 319 is the first riser drilling operation within scientific ocean drilling. Operations included riser drilling at Site C0009 in the forearc basin above the plate boundary fault, non-riser drilling at Site C0010 across the shallow part of the megasplay fault system – which may slip during plate boundary earthquakes – and initial drilling at Site C0011 (incoming oceanic plate) for Expedition 322. At Site C0009, new methods were tested, including analysis of drill mud cuttings and gas, and in situ measurements of stress, pore pressure, and permeability. These results, in conjunction with earlier drilling, will provide (a) the history of forearc basin development (including links to growth of the megasplay fault system and modern prism), (b) the first in situ hydrological measurements of the plate boundary hanging wall, and (c) integration of in situ stress measurements (orientation and magnitude) across the forearc and with depth. A vertical seismic profile (VSP) experiment provides improved constraints on the deeper structure of the subduction zone. At Site C0010, logging-while-drilling measurements indicate significant changes in fault zone and hanging wall properties over short (< 5 km) along-strike distances, suggesting different burial and/or uplift history. The first borehole observatory instruments were installed at Site C0010 to monitor pressure and temperature within the megasplay fault zone, and methods of deployment of more complex observatory instruments were tested for future operations. doi:10.2204/iodp.sd.10.01.2010

IODP Expedition 319, NanTroSEIZE Stage 2: First IODP Riser Drilling Operations and Observatory Installation Towards Understanding Subduction Zone Seismogenesis

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a major drilling project designed to investigate fault mechanics and the seismogenic behavior of subduction zone plate boundaries. Expedition 319 is the first riser drilling operation within scientific ocean drilling. Operations included riser drilling at Site C0009 in the forearc basin above the plate boundary fault, non-riser drilling at Site C0010 across the shallow part of the megasplay fault system &amp;ndash; which may slip during plate boundary earthquakes &amp;ndash; and initial drilling at Site C0011 (incoming oceanic plate) for Expedition 322. At Site C0009, new methods were tested, including analysis of drill mud cuttings and gas, and &lt;i&gt;in situ&lt;/i&gt; measurements of stress, pore pressure, and permeability. These results, in conjunction with earlier drilling, will provide (a) the history of forearc basin development (including links to growth of the megasplay fault system and modern prism), (b) the first &lt;i&gt;in situ&lt;/i&gt; hydrological measurements of the plate boundary hanging wall, and (c) integration of &lt;i&gt;in situ&lt;/i&gt; stress measurements (orientation and magnitude) across the forearc and with depth. A vertical seismic profile (VSP) experiment provides improved constraints on the deeper structure of the subduction zone. At Site C0010, logging-while-drilling measurements indicate significant changes in fault zone and hanging wall properties over short (&lt; 5 km) along-strike distances, suggesting different burial and/or uplift history. The first borehole observatory instruments were installed at Site C0010 to monitor pressure and temperature within the megasplay fault zone, and methods of deployment of more complex observatory instruments were tested for future operations. &lt;br&gt;&lt;br&gt; doi:&lt;a href="http://dx.doi.org/10.2204/iodp.sd.10.01.2010" target="_blank"&gt;10.2204/iodp.sd.10.01.2010&lt;/a&gt;

Possible strain partitioning structure between the Kumano fore‐arc basin and the slope of the Nankai Trough accretionary prism

A 12 km wide, 56 km long, three‐dimensional (3‐D) seismic volume acquired over the Nankai Trough offshore the Kii Peninsula, Japan, images the accretionary prism, fore‐arc basin, and subducting Philippine Sea Plate. We have analyzed an unusual, trench‐parallel depression (a “notch”) along the seaward edge of the fore‐arc Kumano Basin, just landward of the megasplay fault system. This bathymetric feature varies along strike, from a single, steep‐walled, ∼3.5 km wide notch in the northeast to a broader, ∼5 km wide zone with several shallower linear depressions in the southwest. Below the notch we found both vertical faults and faults which dip toward the central axis of the depression. Dipping faults appear to have normal offset, consistent with the extension required to form a bathymetric low. Some of these dipping faults may join the central vertical fault(s) at depth, creating apparent flower structures. Offset on the vertical faults is difficult to determine, but the along‐strike geometry of these faults makes predominantly normal or thrust motion unlikely. We conclude, therefore, that the notch feature is the bathymetric expression of a transtensional fault system. By considering only the along‐strike variability of the megasplay fault, we could not explain a transform feature at the scale of the notch. Strike‐slip faulting at the seaward edge of fore‐arc basins is also observed in Sumatra and is there attributed to strain partitioning due to oblique convergence. The wedge and décollement strength variations which control the location of the fore‐arc basins may therefore play a role in the position where an along‐strike component of strain is localized. While the obliquity of convergence in the Nankai Trough is comparatively small (∼15°), we believe it generated the Kumano Basin Edge Fault Zone, which has implications for interpreting local measured stress orientations and suggests potential locations for strain‐partitioning‐related deformation in other subduction zones.

A low-velocity zone with weak reflectivity along the Nankai subduction zone

Three-dimensional seismic reflection data reveal the presence of a low seismic velocity zone (LVZ) with weak reflectivity character along the Nankai accretionary prism. This LVZ is intercalated between an upper, offscraped layer and a lower, underthrusting layer in the outer accretionary wedge. Wide-angle ocean bottom seismograph data also support the presence of the LVZ, which is estimated to be a maximum of ∼2 km thick, ∼15 km wide, and ∼120 km long. The LVZ could be an underthrust package underplated in response to the lateral growth of the Nankai accretionary prism. Underplating of the underthrusting layer beneath the overlying offscraped layer would maintain a critical taper of the accretionary prism so that the offscraped layer can continue to grow seaward. The LVZ could have elevated fluid pressure, leading to rigidity reduction of the entire outer accretionary wedge. The rigidity-lowered outer wedge, containing the LVZ, may be more easily uplifted and thus eventually foster tsunami generation during a Nankai megathrust earthquake. If the fluid-rich LVZ supplies a significant amount of the fluid to the megasplay fault zone at depth, it may affect stick-slip behavior of the fault.

Petrophysically determined lithofacies at the Nankai Trough Accretionary Prism: NanTroSEIZE, IODP Expedition 314

Abstract: Characterizing the physical properties and identifying boundaries within active accretionary prisms is necessary in understanding their behaviour and recent movement. In such unstable conditions core recovery is not always reliable, especially around fault zones. Integrated Ocean Drilling Program (IODP) Expedition 314 was the first stage of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), and used logging-while-drilling (LWD) technology to record continuous physical property data. We use iterative non-hierarchical cluster analysis (INCA) to quantitatively define the characteristics of the slope sediments and sediments within the accretionary prism at Sites C0001 and C0004. A new and detailed log-based lithostratigraphy is developed, and positions of major boundaries, defined by 3D seismic profiles and initial interpretation of log responses, are refined. The results produce clusters that clearly distinguish the slope sediments and characterize formations within the accretionary prism. Boundaries that correlate to the seismic-defined unconformity between the slope sediments and the accretionary prism, and a boundary within the accretionary prism that corresponds to a megasplay fault previously unresolved by log analysis and borehole images, are identified. Our study demonstrates that INCA analysis of LWD data can accurately define boundaries and characterize sediments in environments where core recovery may be incomplete.

Estimation of temperature rise in a shallow slip zone of the megasplay fault in the Nankai Trough

A megasplay fault branching from the subduction boundary megathrust in the Nankai Trough is thought to have caused large tsunamis associated with past Tonankai earthquakes. Because the temperature recorded in the fault can constrain parameters of seismic slip, we evaluated several temperature proxies (fluid-mobile trace element concentrations, Sr isotopes, magnetic minerals, inorganic carbon content, and Raman spectra of carbonaceous material) in material from a localized slip zone, sampled at Site C0004 by Integrated Ocean Drilling Program Expedition 316. Our results showed that the shallow part of the megasplay fault has not experienced temperatures above 300 °C. We also found that in addition to shear stress and slip distance, slip rate and porosity are effective parameters for frictional heat, as demonstrated by numerical analysis incorporating thermal diffusion. The slip rate estimated through these proxies may indicate that the fault did not slip with high velocity near the seafloor.

Broad, weak regions of the Nankai Megathrust and implications for shallow coseismic slip

Deep within the Nankai Trough subduction zone the plate-boundary thrust slips along a well-imaged megasplay fault system during the megathrust earthquakes that regularly strike southwest Japan. The routing of the active plate-boundary thrust along an upward-branching splay fault causes deep underthrusting of an unusually thick section of material attached to the subducting ocean crust. Here we present three-dimensional seismic reflection data that shows this unusually thick section is fluid-rich sediment that results in broad, weakly-coupled regions of the megathrust down into the updip end of the seismogenic zone. The weakly coupled regions lie above an underthrust low seismic impedance (presumed to be low-velocity and low-density) sediment section that is between one and two kilometers thick and covers ~ 3300 km 2, at least one-eighth of the total rupture area of the 1944 Mw 8.1 Tonankai earthquake. This underthrust sediment section is broader and much deeper than inferred at other margins. Sediment underthrusting into the normally well-coupled seismogenic zone likely releases fluids and elevates fluid pressure, which reduces inter-plate coupling along portions of the megasplay fault and allows coseismic rupture to propagate to unusually shallow depths and generate large tsunami as inferred for the 1944 Tonankai event. Splay faults may be a common, yet transient mechanism for developing weak subduction zone thrusts.

Three-Dimensional Splay Fault Geometry and Implications for Tsunami Generation

Megasplay faults, very long thrust faults that rise from the subduction plate boundary megathrust and intersect the sea floor at the landward edge of the accretionary prism, are thought to play a role in tsunami genesis. We imaged a megasplay thrust system along the Nankai Trough in three dimensions, which allowed us to map the splay fault geometry and its lateral continuity. The megasplay is continuous from the main plate interface fault upwards to the sea floor, where it cuts older thrust slices of the frontal accretionary prism. The thrust geometry and evidence of large-scale slumping of surficial sediments show that the fault is active and that the activity has evolved toward the landward direction with time, contrary to the usual seaward progression of accretionary thrusts. The megasplay fault has progressively steepened, substantially increasing the potential for vertical uplift of the sea floor with slip. We conclude that slip on the megasplay fault most likely contributed to generating devastating historic tsunamis, such as the 1944 moment magnitude 8.1 Tonankai event, and it is this geometry that makes this margin and others like it particularly prone to tsunami genesis.

Thermal process reduces nitrous oxide emissions by more than 90%

Although redox state plays an important role in chemical reactions, it has received little attention in terms of processes of fluid–rock interaction in fault zones. The results of geochemical analyses (major element mapping and trace element analyses) of carbonate veins along the Nobeoka Thrust, a fossilized megasplay fault within the Shimanto Accretionary Complex, Japan, suggest dynamic sudden change in redox state associated with seismic faulting and fluid flow. Fluids migrating along the fault zone were relatively oxidized during the pre-failure fluid pressurization stage of the seismic cycle, but were relatively reduced during the immediate post-failure stage. The generation of a reducing post-failure hydrothermal regime may be associated with seismogenic rupture along the megasplay fault, involving the advection of exotic reducing fluid from deeper levels or in situ generation of hydrogen by co-seismic mechanochemical reactions.